HVAC system efficiency increases have provided considerable reductions in energy use. For example, many high efficiency furnaces, air conditioners, and air handlers now have efficiencies (AFUE ratings) greater than 90%. However, the blower motors used to move the air in these systems have not seen significant efficiency improvements and have much lower efficiencies. As furnaces and air conditioners have become more efficient, the fraction of total energy consumption for HVAC systems attributed to the blower motors has increased, thus making blower motors a greater contributor to the overall system energy use.
Blower motor inefficiencies are magnified when a blower motor is operated for extended hours beyond that needed solely for heating and cooling. For example, some users frequently choose to let their HVAC system's blower motor operate continuously by setting a fan control switch to the “on” position. This circulation mode of operation reduces temperature stratification, minimizes start drafts from duct work, improves humidity control, and increases the effectiveness of associated air cleaners employed in conjunction with the HVAC system. By selecting the “on” position, the blower motor operates continuously, and the associated thermal feature, (i.e., either heating or cooling) operates on the “demand” setting of the thermostat. When in the “on” position, the blower motor typically operates at the speed used for cooling, even when the thermostat is set to heat mode. This speed is usually well in excess of what is necessary to achieve the air circulation benefits outlined above. This causes excess energy usage and noise. In addition, with the blower switch in the “on” position, the unit no longer is able to select the system speed for cooling or heating and instead continuously runs at the continuous fan speed. Even when systems are designed to select the proper speed in a multiple speed motor, for example, as disclosed in U.S. Pat. No. 4,815,524, the speed available for blower “on” use is higher than necessary for such operations, and can be responsible for cold spot corrosion, requiring a shut down period disclosed in the '524 patent. The increased operation time therefore leads to greater energy use.
Many of the above-described inefficiencies result from the type of blower motor used in HVAC systems. HVAC systems traditionally use fixed speed or multiple speed permanent split capacitor (PSC) motors. These motors generally have two independent power connections to accommodate heating or cooling modes of operation. The heating or cooling power inputs are normally connected to different winding taps in the PSC motor to provide somewhat different operating speeds for the blower in the respective modes of operation. More than two taps can be designed into the PSC motor, allowing the OEM or installer to select the operating speed by appropriate connection of the taps to the respective heating and cooling power connections. The energizing of these AC power connections to the motor is controlled by activation of a temperature switch and a relay driven from the thermostat.
An example of a fixed speed PSC motor M1 used in residential HVAC systems is shown in FIG. 1. In this configuration, the single phase AC supply voltage (normally 115 VAC or 230 VAC) is supplied by connections L1 and N, where L1 represents the hot side of the AC supply, and N is neutral, which is at earth potential in a typical 115 VAC residential distribution system. (In normal 230 VAC systems, another hot supply line would be substituted for the neutral line N.) The power to the motor is controlled by a relay R1 and a switch S1, which are both shown in their non-energized positions. The blower relay R1 is controlled by a thermostat.
In the position shown in FIG. 1, which is the normal position for the heating mode of operation, AC voltage is supplied to a power input connection L1H motor connection any time fan control switch S1 closes. The fan control switch S1 closes whenever the air temperature in the heat exchanger exceeds a predetermined setpoint. For a gas furnace system, this happens a short time after the gas burner is activated by signals from the thermostat once the thermostat reaches a trigger temperature. When fan control switch S1 closes, AC power is supplied to the motor M1, which will then start and run. The speed of motor M1 is a function of motor design, tap selection in the motor, blower characteristics and the aerodynamic system impedance. Motor M1 stops when fan control switch S1 turns off, which happens whenever the heat exchanger air temperature decreases below the setpoint.
Similarly, when the thermostat demands blower operation because of cooling demand, blower relay R1 closes and energizes the L1C motor connection, thus operating the motor at its cooling mode speed. Blower operation ceases when signals from the thermostat de-energize blower relay R1.
Referring now to FIG. 1A, another fixed speed PSC motor used in residential HVAC systems is shown. The motor has four winding taps to accommodate two heating fan speeds and two cooling speeds. The fan speed is controlled by a furnace control board with a cool/heat relay, a low/high cool relay, and a low/high heat relay. Other HVAC systems may include two heating stages and a single cooling stage or any other combination of heating and cooling speeds.
PSC motors are reasonably efficient when operated at high speed, but their efficiencies may drop down into the 20% range when operated at low speeds. Because air conditioner evaporator coils need higher airflow than furnace heat exchangers, the blower motor operates at a lower speed during furnace operation, where it is less efficient, and at an even lower speed still during continuous fan “on” operation, where it is least efficient.
Because of the above-described inefficiencies of PSC motors, many newer HVAC systems use variable speed motors such as brushless permanent magnet (BPM) motors and corresponding electronic variable speed motor controllers. The speed of a BPM can be electronically controlled and set specifically to match the airflow requirements for each application, thus permitting more efficient operation. Also, BPM motors use power approximately proportional to the cube of motor speed, whereas PSC motors use power approximately proportional to motor speed. therefore, as motor speed drops, BPM motors use less power than PSC motors over a wide range of motor speeds. This is particularly important when operating the blower continuously for circulation as described above.
While variable speed motors are often superior to PSC motors, replacing an existing PSC motor with a variable speed motor in a system similar to that illustrated in FIG. 1A has required costly, time-consuming, and complex changes in the mechanical, wiring, or control configuration of the system. Variable speed motor systems configured for replacement of PSC motors in existing HVAC systems have been developed, but they have relatively complicated control and sensing systems. For example, some systems require the installation of a temperature sensor in the outlet ductwork of the HVAC system for controlling the speed of the motor based upon temperature. In other replacement systems, the installation of a replacement motor requires continuous power connection to the motor and the connection of low voltage control signals directly from the thermostat to the motor. Making these connections can be cumbersome and difficult in an existing HVAC system. Moreover, these known systems lack the sensitivity to operate blowers at low operating speeds.
Another limitation of existing PSC and BPM motors is that HVAC OEMs often require motors with unique operating parameters (torque load, fan speed, rotation direction, etc.) to optimize the performance of their HVAC components. While multiple speed PSC motors and BPM motors offer some operational options, many of their operating parameters are fixed after manufacture and cannot be easily changed. Motor manufacturers, installers, and service contractors therefore must stock a diverse inventory of blower motors to accommodate the various different models of HVAC equipment.
It would therefore be desirable to provide an improved “drop-in” replacement for a PSC motor in an HVAC system to realize the advantages of a variable speed blower motor without requiring significant changes to the HVAC system. It would be further advantageous to reduce the complexity of such replacement systems by utilizing simple control circuits and eliminating the need for additional wiring, such as that used in conjunction with traditional variable speed motors and existing replacement variable speed motor systems. It would also be advantageous to provide an HVAC blower motor that could be customized to accommodate more HVAC systems.